Surrey police are evacuating neighbours of the murdered engineer Saad al-Hilli after finding "items" at the family home in Claygate, Surrey.

Neighbours living close to the property were prevented from returning to their homes as army bomb disposal officers were called to the scene. The police investigation appeared to focus on a shed or workshop at the bottom of the garden.

Hilli was shot in the head last week along with his wife, Iqbal, and a woman police have now confirmed was his mother-in-law while on a camping holiday with their two daughters in Annecy, in the French Alps. The children, Zainab, seven, and Zeena, four, survived the attack, which took place while the family were in their car.

A witness who was cycling past was also shot in the head. French authorities leading the investigation into the multiple murders have suggested the attack bore the hallmarks of a professional killing.

On Monday, Surrey police said they were evacuating homes in the village of Claygate after finding items that concerned them. The £1.5m mock Tudor home has been under police guard since Thursday, and police and forensic teams began searching the house this weekend for clues that could help French police find the killers.

A Royal Logistic Corps bomb disposal truck arrived shortly after the police cordon was extended, at around 9.30am. Officers were seen apparently receiving a briefing at a neighbouring home.

British police are working alongside their French counterparts, trying to unravel the mystery of the four deaths.

Officers first entered the Hillis' family home on Saturday after a team of four French investigators, led by Colonel Marc de Tarle, arrived in the UK.

In a short statement, the force said: "Surrey police can confirm that due to concerns around items found at the address in Oaken Lane in Claygate, officers have extended the cordon surrounding the property.

"Neighbours in the immediate area are being temporarily evacuated and we will provide more information when we have it."

French police have confirmed that the elderly woman found murdered in the car was Iqbal al-Hilli's mother.

Detectives had struggled to identify the 74-year-old Swedish passport-holder. Zeena, who survived by hiding underneath her dead mother's skirt for eight hours, was able to confirm her parents' identity, telling investigators who showed her photographs: "That's Mummy … and that's Daddy." However, when asked about the older woman, the traumatised child told them it was someone she had not seen often, and of whose identity she was uncertain.

Zainab suffered severe head injuries in the attack and underwent two operations in Grenoble University hospital, where she was in a medically induced coma until Sunday afternoon.

On Monday, the French investigating team said a weekend search of the family home had revealed the woman was the girls' grandmother.

In Annecy, police are examining a laptop computer and two mobile phones found in the family car.

As speculation continued over possible motives for the murders, one man, thought to be one of the last people to see the family alive, is reported to have told journalists the family had arrived at the isolated car park at least one hour before they were killed.

Laurent Fillion-Robin, 38, also said there was no sign of any vehicle following the family. The builder said he had been working on a house in Chevaline when he saw the red British-registered BMW drive past, between 2.30pm and 3pm. The shooting was reported to police just before 4pm.